Ice sheet interaction with the global climate system
As global sea level continues to rise at an accelerating rate, our ice-sheet modelling research focuses on better understanding how the Antarctic and Greenland ice sheets are contributing to this rise, and how they might respond to future environmental changes.
Whilst the West Antarctic Ice Sheet is known to be the most vulnerable to climate warming, due to its bed lying below present sea level, it is the East Antarctic Ice Sheet (EAIS) that holds the greatest volume of ice - about 53 m of sea-level equivalent water, ten times that of West Antarctica. In the ARC, our approach of combining fieldwork and computer modelling to quantify how the EAIS changed during past climate transitions is immensely powerful, and the use of the two methods together allows us to make robust predictions for the future.
So why do global climate models nearly always underestimate the amount of natural variability in the climate system? This is an important problem, because it is precisely these models that are used to make predictions about future climate changes and which in turn are used to guide government policy. So utilising simulations performed by the ARC’s Nick Golledge looked at global climate variability through the Holocene - the period of geological time that spans the last 10,000 years. The results, published in Nature, were remarkable, and found that even very small amounts of meltwater from Antarctica, discharged into the ocean, were sufficient to change the way that the ocean mixes and transports heat. The knock-on effect is that these changes in oceanic heat transfer in turn influence the temperature of the overlying air masses. The simulations also illustrated a previously-known ‘seesaw’ effect, in which the release of meltwater in Antarctica leads to cooling of the surface water around the ice sheet, and a cooling of air temperatures across the Southern Hemisphere, but a warming of surface temperatures in the Northern Hemisphere. This study therefore not only explained the climate variability problem that was the original goal, but also led to new insights into how future meltwater from Antarctica might affect global climate over coming decades to centuries.
In a separate study published in Geophysical Research Letters, Nick used his ice sheet model to investigate the differing roles played by atmosphere, ocean, and bedrock topography in controlling the long-term evolution of the Antarctic ice sheets. Nick ran simulations for the whole continent and analysed the results on a catchment-by-catchment basis. What was immediately striking was that different sectors of the West and East Antarctic ice sheets respond in very different ways, and perhaps more importantly, they each respond preferentially to different drivers. The simulations showed, as expected, that West Antarctic drainage basins were most sensitive to ocean warming, but what was less expected was that whilst the majority of East Antarctica is sensitive mainly to a warming atmosphere, there are one or two
catchments that are uniquely sensitive to the ocean. Of these, the Recovery Basin catchment, in the eastern Weddell Sea, appeared to be the most sensitive, and in fact behaved in many ways like a West Antarctic system. The significance of this finding is that the Recovery Basin is currently an area that has received very little attention, being remote and difficult to access. But observational and modelling data are increasingly beginning to show that climate-driven changes are occurring
in this area, and that these changes may lead to significant retreat of part of the ice sheet there. By contrast, sectors such as the Totten Glacier have attracted more attention over recent years because measured ocean warming there is considered to pose a threat to the stability of what is a large and important glacier. But this new analysis suggests that the Totten Glacier sector of the ice sheet is buffered from ocean-driven retreat by the topographic configuration of its bed. Essentially, the glacier rests on a complex landscape of hills and valleys, rather than a single deep basin, so any retreat of the glacier in this area will be relatively slow.
Together, these and other studies are revealing more and more about the complexity of ice sheet behaviour and how ice sheets interact with the global climate system. By continually refining these computer models, and gathering the field data to calibrate them, the ARC is contributing to an increasing global effort to better predict the future of our global ice sheets.
For more information contact: Nick Golledge